(HealthDay News) -- Fentanyl, a synthetic narcotic, is a key player in America's continuing epidemic of opioid-related overdose deaths, two new studies report.

Opioid OD deaths began increasing early this decade as people addicted to prescription painkillers started switching to heroin, which became cheaper and increased in supply, researchers from the U.S. Centers for Disease Control and Prevention said.

But the emergence of fentanyl and its cousins drove an even greater increase in overdose deaths, exacerbating an already terrible situation, the researchers said.

In 2015 alone, about 33,000 people died of an opioid overdose in the United States.

When drug traffickers began mixing fentanyl into the heroin supply, they created a highly potent narcotic cocktail that has proven fatal to thousands of drug users, investigators noted.

"Approximately half of the increase in deaths involving heroin after 2013 is attributable to increases in deaths involving use of both heroin and fentanyl," researchers with the CDC's National Center for Injury Prevention and Control wrote in one of the studies.

But deaths from fentanyl-type drugs with no heroin involvement also increased substantially in recent years.

About 90 percent of OD deaths in Ohio early this year involved fentanyl or its chemical cousins, state public health officials reported in the second paper. Only 6 percent of overdoses involved heroin.

In essence, the U.S. opioid epidemic has progressed to a new and more dangerous wave, said Dr. Harshal Kirane, director of addiction services at Staten Island University Hospital in New York City.

Prescription drug abuse represented the first wave, followed by the switch to heroin, said Kirane. Heroin began to be cut with fentanyl, and now people are seeking out and using fentanyl on its own.

"This data suggests that synthetic opioids now are being used as the primary substance itself," Kirane said. "That represents another tragic step in our opioid crisis."

Fentanyl is 50 to 100 times more potent than morphine, making it much more powerful than either heroin or prescription opioids, according to the U.S. National Institute on Drug Abuse.

If someone doesn't know the heroin they just bought is laced with fentanyl, it's easy to accidentally overdose.

"Many people are unaware that they may be risking their life when they think they are simply buying heroin to support their ongoing prescription drug habit," said Dr. Robert Glatter, an emergency physician with Lenox Hill Hospital in New York City.

After 2013, deaths caused by the combined use of heroin and synthetic opioids accounted for 77 percent of the total increase in heroin-related deaths in the U.S. Northeast and Midwest, the two regions with the sharpest increases in fentanyl use, CDC researchers said.

The emergence of ever-stronger fentanyl analogues poses an even greater risk. Carfentanyl, in particular, is one of the most potent cousins to fentanyl, even more powerful than the original substance, Kirane said.

"People who buy fentanyl online are playing Russian roulette," Glatter said. "These drugs are incredibly powerful in the smallest amounts and can kill you instantly."

Synthetic opioids are so powerful that naloxone, the overdose-reversal medication, can be rendered ineffective, Kirane said. Naloxone (Narcan) may not work at all, or the person may need more naloxone than is on hand to counter an overdose.

The nation continues to lose ground in its fight against opioids because it hasn't tackled the root issue of addiction, said Lindsey Vuolo, associate director of health law and policy at the National Center on Addiction and Substance Abuse.

"We don't have a significantly higher number of people who are getting treatment. That is the major problem," Vuolo said. "We have to expand access to evidence-based treatment."

FDA-approved medications for opioid addiction like methadone and buprenorphine protect against overdose, Vuolo said. On the other hand, people not in treatment will continue to use drugs and will be exposed to ever-more-potent synthetic opioids.

Results from both studies appear in the CDC's Aug. 31 Morbidity and Mortality Weekly Report.

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